1. Field of the Invention
The present invention relates to a rolling bearing unit fitted with a rotational speed sensor (referred to hereunder as a speed sensing rolling bearing unit), used for rotatably supporting a vehicle wheel on a suspension unit, and detecting the rotational speed of the wheel.
2. Description of the Related Art
Referring to FIG. 8, there is shown an example of a speed sensing rolling bearing unit as disclosed in U.S. Pat. No. 4,907,445, for rotatably supporting a vehicle wheel on a suspension unit, and detecting the rotational speed of the wheel in order to control of an anti-lock braking system (ABS) or a traction control system (TCS).
The speed sensing rolling bearing unit shown in FIG. 8, has a hub 3 having a flange 1 for wheel fixture provided on an axially outer end portion (the term "axially outer" means the widthwise outer side when fitted to a vehicle; the left side in FIG. 8) and a first inner ring raceway 2a formed on a central outer peripheral face thereof, and an inner ring 4 having a second inner ring raceway 2b formed on an outer peripheral face thereof, and externally fitted to the outer peripheral face of the central portion of the hub 3. The inner ring 4 constitutes an inner ring assembly together with the hub 3.
Formed on an outer peripheral face of the axially inner end portion of the hub 3 (the term "axially outer" means the widthwise outer side when fitted to a vehicle; the left side in FIG. 8) is a threaded portion 5, onto which a nut 6 is threaded to thus press against the axially inner end face of the inner ring member 4, and thereby secure the inner ring member 4 at a predetermined location on the outer peripheral face of the hub 3. The speed sensing rolling bearing unit has also an outer ring member which has an attachment portion 7 on an outer peripheral face thereof for attachment to a suspension unit (not shown) and a double row outer ring raceway 8a, 8b formed on an inner peripheral face thereof.
A plurality of rolling elements 10 are provided between the outer ring raceway 8a, 8b and the first and second inner ring raceways 2a, 2b, respectively, so that the hub 3 for wheel fixture is rotatably supported inside the outer ring member 9 which is in turn supported on the suspension unit.
The speed sensing rolling bearing unit further has a cylindrical tone wheel 11 which is externally secured to an axially inner half portion of the inner ring 4. Recesses/protrusions 12 are formed on the axially inner end face of the tone wheel 11, so that the magnetic characteristics of the axially inner end face are changed alternately in the circumferential direction and at an even spacing.
The outer ring member 9 is formed with an opening at its axially inner end, and the cover 13 is mounted in an opening portion at the inner end of the outer ring member 9 to cover the opening. The speed sensing rolling bearing unit further has a sensor 14 which is fixed to the cover 13, such that an axially outer end face of the sensor 14 is opposed to the recesses/protrusions 12 of the tone wheel 11.
At the time of operation of the speed sensing rolling bearing unit as described above, a vehicle wheel fixed to the flange 1 on the axially outer end of the hub 3 is rotatably supported relative to the suspension unit to which the outer ring member 9 is attached. When the tone wheel 11, externally secured to the inner ring member 4, rotates with rotation of the vehicle wheel, the output of the sensor 14 faced to the recesses/protrusions 12 on the inner end face of the tone wheel 11 changes. Since the frequency with which the output of the sensor 14 changes is proportional to the rotational speed of the wheel, then if the output signal from the sensor 14 is input to a controller (not shown), the rotational speed of the vehicle wheel can be obtained, so that an anti-lock braking system (ABS) or a traction control system (TCS) can be appropriately controlled.
With the conventional speed sensing rolling bearing unit constructed and operated as described above, miniaturization is difficult, so that vehicles to which it can be fitted are limited to relatively large vehicles. More specifically, in order to detect the rotational speed of the vehicle wheel, the sensor 14 must be arranged so as to be axially faced to the recesses/protrusions 12 formed on the axially inner end face of the tone wheel 11. Therefore a large axial dimension for the rotational speed detection part cannot be avoided.
In particular, in the case of comparatively low cost units, a simple magnetic material member (not permanent magnet material) is used for the tone wheel 11, and a so-called passive type device with a coil wrapped around a core member of magnetic material is used for the sensor 14 faced to the tone wheel 11. With such a passive type sensor 14, in order to maintain enough the output of sensor 14, the axial dimension of the sensor 14 is increased to some extent. Accordingly, there are problems depending on the extent that the axial dimensions of the rotational speed detection unit are increased.
As well as the abovementioned speed sensing rolling bearing units, there are also those disclosed in Japanese Patent First Publication KOKAI No. H1-175502, and Japanese Utility Model First Publication KOKAI No. H3-99676. With the configurations disclosed in these respective publications, the magnetic characteristics of the inner and outer peripheral faces of the tone wheel are changed around the peripheral direction, and the detection portion of the sensor is arranged to be faced to the inner and outer peripheral faces. With such configurations, since the core member of the passive type sensor is positioned in the axial direction, sufficient miniaturization is not always possible.
European Patent Publication No. 0426298A1 discloses a configuration wherein the limited space is effectively utilized and the sensor output increased, by forming the sensor in an annular shape so that an inner peripheral face of the sensor is radially opposed to an outer peripheral face of the tone wheel around the whole circumference. With the speed sensing rolling bearing unit according to this European Patent Publication, as shown in FIG. 9, gear tooth-like projections are formed around the outer peripheral face of a tone wheel 11a which is fixed to an axially inner end of a hub 3. Moreover an annular sensor 14a is retained inside an inner peripheral face of a cover 13 which is secured to an inner end opening portion of an outer ring member 9, so that an inner peripheral face of the sensor 14a is radially opposed to an outer peripheral face of the tone wheel 11a across a small gap, around the whole circumference.
By forming the sensor 14a in an annular shape so that the inner peripheral face of the sensor 14a is opposed to the outer peripheral face of the tone wheel 11 around the whole circumference, then the amount of change in the magnetic flux flowing in the sensor 14a can be increased, and the output of the sensor 14a thus increased. As well enabling an increase in the output of the sensor 14a, the annular shape of the sensor 14a enables the limited space to be effectively utilized, so that the speed sensing rolling bearing unit is not increased in size.
With the speed sensing rolling bearing unit of the abovementioned European patent publication, the following points still require improvement. That is to say, due to vibrations accompanying vehicle travel, the sensor 14a is subjected to forces in a direction to displace the sensor 14a from the cover 13. Moreover when such forces act on a joint between the sensor 14a and the cover 13, this joint is damaged. For example, the sensor 14a comprising a permanent magnet, a pole piece and a coil formed in annular shape, and embedded in a synthetic resin, is fitted into the cover 13. This is a primary synthetic resin molding. A connector into which is inserted a plug on the end of a signal output harness, is then molded from synthetic resin, and this is a secondary synthetic resin molding. And the primary synthetic resin molding for the main body 25 welded to the secondary synthetic resin molding for the connector via an aperture formed in the cover 13. Therefore, if the sensor 14a vibrates, a stress acts on this welded joint.
If this stress is small there is no real problem, however the stress can becomes large proportional to the mass of the sensor 14a. Moreover, while the annular sensor 14a can be located in a limited space, the overall weight is increased. As a result, with a speed sensing rolling bearing unit fitted with the annular sensor 14a, the welded joint of the abovementioned main body portion and connector portion is susceptible to damage.